To COI is to "cum on impact". A problem every man has or eventually will have. When your penis touches vagina for the first time in a while, no matter how much hand action it's been seeing and the vagina's touch is just too much. You explode into her after just seconds of penetration. A COI is usually categorized between 15 and 1 second of intercourse yet for the more experienced it may be more of a 1 to 2 minute long voyage (when you're somewhat experienced and can normally last like 10 or 15 minutes at least).

Either way, you never want to COI with anyone other then someone you've already given good sex to, otherwise they probably won't ever want to do it again, and will tell all their friends (and yours) about your inability to squeeze back your load for more then 5 seconds.

"Dude! Sarah was on her period last week but she's finally off so we can finally fuck!"